1. Field of the Invention
The present invention relates to an apparatus and method for digital wireless communications using a multivalue modulation type.
2. Description of the Related Art
In a conventional digital mobile wireless communication system, a familiar example of frame configuration method to estimate a frequency offset is described in xe2x80x9cTerrestrial Mobile Communication 16 QAM Fading Distortion Compensation Methodxe2x80x9d (Sanbe, TECHNICAL REPORT OF IEICE, B-II, Vol. J-72-B-II, No. 1, PP.7-15, Jan. 1989). FIG. 1 shows a frame configuration according to a 16 QAM system.
As shown in FIG. 1, this frame configuration has one pilot symbol inserted for every N-1 information symbols. With such a frame configuration, quasi-coherent detection is performed by estimating the reference phase, amount of frequency offset and amount of amplitude distortion using pilot symbols.
However, during quasi-coherent detection with such a frame configuration with one pilot symbol inserted for every few information symbols, symbol synchronization gets the jitters. Therefore, in quasi-coherent detection with symbols whose symbol synchronization is not completely established, the accuracy in estimating the reference phase, amount of frequency offset and amount of amplitude distortion using pilot symbols deteriorates. This results in deterioration of a bit error rate characteristic in the signal to noise ratio.
This is explained more specifically using FIG. 2A and FIG. 2B. FIG. 2A and FIG. 2B are diagrams to explain the relationship between the time and amplitude of a reception signal. In FIG. 2, reference code 1 indicates the time when pilot symbol 3 is detected with an ideal judgment time and reference code 2 indicates the time when pilot symbol 3 is detected with a time offset (jitter) generated. Reference code 4 indicates the information symbols immediately before and after pilot symbol 3.
Both a transmitter and receiver are provided with their respective clock generation functions. Because of this, the receiver has different clock generation sources, and therefore the receiver may detect waves at timing such as time 2, at which a time offset from ideal judgment time 1 has occurred. At this time, as shown in FIG. 2A and FIG. 2B, the time offset originates errors (amplitude errors) XI and XQ respectively, from the signal point. This deteriorates the error rate. Furthermore, the receiver estimates the phase, amplitude variation and frequency offset on the I-Q plane from the pilot symbol. However, when detected at time 2 when the time offset occurred, the pilot symbol signal has an error from the pilot symbol signal point, and therefore the accuracy in estimating the phase, amplitude variation and frequency offset deteriorates.
It is an objective of the present invention to provide an apparatus and method for digital wireless communications capable of improving the accuracy in estimating the reference phase and amount of frequency offset when the receiver (demodulation side) carries out quasi-coherent detection and improving the bit error rate characteristic in the signal to noise ratio.
This objective is achieved by a digital wireless communication apparatus that uses a modulation type including QPSK modulation and modulates the signal points of each one symbol immediately before and after a pilot symbol using a modulation type different from the modulation type for the pilot symbol in a frame configuration with one pilot symbol inserted for every 3 or more symbols.
This makes it possible to suppress deterioration of the accuracy in estimating the reference phase and amount of frequency offset using pilot symbols in quasi-coherent detection with symbols whose symbol synchronization is not completely established and improve the bit error rate characteristic in the signal to noise ratio.
Furthermore, this objective is also achieved by a digital wireless communication apparatus that increases the amplitude at pilot symbol signal points more than the maximum amplitude at signal points according to the multivalue modulation type with 8 or more values.
This apparatus can not only suppress deterioration in the accuracy in estimating the reference phase, amount of frequency offset by a pilot symbol in quasi-coherent detection with symbols whose symbol synchronization is not completely established, but also improve the bit error rate characteristic in the signal to noise ratio without deteriorating the power efficiency of the power amplifier on the transmitting side.